configure: use `foo` instead of $(foo)

Seems to work everywhere, whereas $(foo) does not work on
some variants of FreeBSD.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Merge branch 'master' of ssh://brick.kernel.dk/data/git/fio
FIO-VERSION-GEN: use `foo` instead of $(foo)

Signed-off-by: Jens Axboe <axboe@kernel.dk>